Commutating motors and generators are known in industry. Typically, a commutator is attached to an armature of the motor. Commutators comprise a circular array of electrically conducting segments, or commutator bars, that are spaced and electrically insulated from each other. The armature contains numerous windings in a predetermined pattern, each winding being connected to a commutator bar. In a motor, electric current is transferred to the windings by brushes, attached to a current source, that make contact to the commutator bars. As the armature rotates, the brushes contact different bars and conduct current to different windings. The electric field generated thereby interacts with electromagnets located around the armature and produces an electromotive force which enables the motor to rotate. Conversely, in a generator, the armature is rotated by an external source and current produced is conducted to an external load by the brushes as the armature and commutator rotate.
A drawback of commutating motors and generators is brush noise. The noise may be aesthetically unpleasant, and, in an industrial setting, may contribute to a hazardous noisy environment. A primary cause of brush noise is the impact of the brush with the commutator bars as they rotate into contact with the brush. Brushes are positively biased toward the commutator to ensure electrical contact, resulting in impact as the brush transitions from the space between the commutator bars to the bars and also as the brush moves over surface variations in the bar itself. The brush impact with the commutator bar and surface variations may also cause the brush to "bounce" off the surface of the commutator and break the electrical contact between the brush and commutator. This not only decreases performance, but may also cause arcing--the discharge of electricity across the gap between the brush and commutator--which is an additional noise source.
Several methods are known to reduce brush noise. One approach has been to control bar to bar variations so that the surfaces of the commutator bars are positioned at a more uniform radius from the axis of rotation. Although reducing noise created by bar to bar variations, the commutator is more expensive and difficult to manufacture. Some commutators have uniform surfaces where the commutator bars are separated by non-conductive material rather than empty spaces so that there is no distinct edge for the brush to impact. This initially reduces noise, but during service, the materials wear at different rates, forming edges on the commutator bars and ultimately producing noise.
Other known approaches to reduce brush noise attempt to control the motion of the brush as it contacts the commutator. For example, one device restricts movement of the brush relative to its brush holder, reducing vibration. Another device utilizes an elastic damping sleeve to dampen the noise produced. These devices are substantially more complicated and more costly to manufacture and assemble than a conventional brush system. Still another brush design allows the brush to pivot on the surface of the commutator. This arrangement, however, allows parts of the brush to repeatedly engage and disengage the commutator, leading to unwanted arcing and performance reduction.
Moreover, various other commutator designs are known. Some of these approaches vary the width of the commutator bars, i.e., vary the times between successive impacts, reducing the fundamental frequency sound resulting from repeated impact of the brush and commutator. However, these designs merely change the character of the noise by broadening the frequency spectrum and do not reduce other sources of noise. Yet another design varies the shape of the edge of the commutator bars so that different parts of the bar impact the brush at different times. This reduces noise by spreading out the impact over time, but results in a loss of motor performance due to inefficient and varying contact between the brush and commutator during rotation.